A Raman spectrometer or Raman spectrophotometer is a device that optically probes the vibrational, rotational, and low-frequency modes of a solid, liquid, or gaseous chemical or material. It can accurately quantify the chemical structure of an unknown substance. A Raman spectrometer typically operates by first illuminating a sample with a single frequency laser in the visible or near-infrared wavelength region. A fraction of the light that scatters from the sample is converted to a higher optical frequency (anti-Stokes shifted), and another fraction is converted to a lower optical frequency (Stokes shifted). The new frequencies of this Stokes and anti-Stokes shifted light (also referred to as Raman-shifted light) correspond to the intrinsic energy levels of the substance being sensed, and they can be used to uniquely identify the chemical or material, as shown in FIG. 1.
Conventional Raman spectrometer systems usually include several distinct sub-systems: (1) a single-frequency excitation source, such as a laser; (2) an optical probe or region where the light interacts with the analyte or unknown chemical of interest; (3) a dichroic mirror or optical filter that blocks the light from the excitation source, letting only the Stokes or anti-Stokes scattered light pass; and (4) a spectrum analyzer or spectrometer that measures the intensity of the Raman shifted light as a function of frequency or wavelength.
The spectrum analyzer typically includes a dispersive element, such as a grating or prism, that disperses the Raman scattered light for detection by a detector array. The measured Raman spectrum typically contains several peaks, the frequency and intensity of which serve as a unique ‘optical fingerprint’ of the chemical being identified. By comparing this spectrum to a database of known Raman spectra, the composition of single chemicals or mixtures of chemicals in the gas, liquid, or solid phase can be determined with high precision.
In biomedical sensing, Raman spectroscopy is also a promising approach for non-invasively detecting critical physiological and biochemical parameters, such as blood glucose, lactate, blood oxygen saturation levels, etc., owing to its superior chemical selectivity and availability of near-infrared light sources with sufficient penetration depth into biological tissues. Wearable non-invasive blood glucose monitoring promises great relief to diabetes patients for glucose control but remains an outstanding challenge despite the development of commercial glucose meters in the past few decades.
Conventional Raman spectroscopy systems are usually benchtop laboratory equipment with large size and high cost. Portable Raman spectrometers have slowly begun to enter the market in the last five years, though their size reduction has relied primarily upon direct miniaturization of discretely-assembled free-space optical components that need to be manually aligned, such as mirrors, beam-splitters, free-space lenses, and free-space grating spectrometers. These discrete optical components typically do not withstand physical shock or vibrations without requiring realignment or recalibration. In addition, these grating spectrometers suffer from poor sensitivity (which is related to the signal-to-noise ratio), have limited spectral resolution (typically to no more than 1024 channels), and are relatively large and heavy.